The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.
Current imaging practice attempts to acquire high image quality throughout a scanned volume, though some focus is now being directed at more patient specific methods of imaging. It is recognized that many imaging tasks only require elevated image quality in smaller volumes while low image quality would be sufficient throughout the remainder of the imaged volume. The development of techniques to perform region of interest (ROI) imaging (see R. Chityala, K. R. Hoffmann, S. Rudin, and D. R. Bednarek, “Region of interest (ROI) computed tomography (CT): Comparison with full field of view (FFOV) and truncated CT for a human head phantom,” Proc. SPIE Physics of Medical Imaging 5745, 583-590 (2005) and C. J. Moore, T. E. Marchant, and A. M. Amer, “Cone beam CT with zonal filters for simultaneous dose reduction, improved target contrast and automated set-up in radiotherapy,” Phys Med Biol 51, 2191-2204 (2006)) are a step towards acquiring images that provide varying image quality through the reconstructed volume. However, there remains a need for further improvements to be made by having the ability to optimally modulate the x-ray fluence patterns applied during imaging in a more patient specific fashion.
Many technologies have been developed for the purpose of improving external beam radiation therapy by imaging patients in the treatment position. These systems, which include CT imagers placed on rails in the treatment room (see K. Kuriyama, H. Onishi, N. Sano, et al. “A new irradiation unit constructed of self-moving gantry-CT and linac,” Int J Radiat Oncol Biol Phys 55, 428-35 (2003)), Tomotherapy (see T. R. Mackie, T. Holmes, S. Swerdloff, et al. “Tomotherapy: a new concept for the delivery of dynamic conformal radiotherapy.” Med Phys 20, 1709-19 (1993)), and imaging CT systems mounted on the gantries of conventional linear accelerators have the potential to improve radiation therapy targeting. One example of a CT imaging system is cone-beam CT (see D. A. Jaffray, J. H. Siewerdsen, J. W. Wong, and A. A. Martinez, “Flat-panel cone-beam computed tomography for image-guided radiation therapy,” Int J Radiat Oncol Biol Phys 53, 1337-1349 (2002)) and another example is scanning-beam CT (see E. G. Solomon, B. P. Wilfley, M. S. Van Lysel, A. W. Joseph, and J. A. Heanue, “Scanning-beam digital x-ray (SBDX) system for cardiac angiography,” in Medical Imaging 1999: Physics of Medical Imaging (SPIE, New York, 1999), Vol. 3659, pp. 246-257; T. G. Schmidt, J Star-Lack, N. R. Bennett, S. R. Mazin, E. G. Solomon, R Fahrig, N. J. Pelc, “A prototype table-top inverse-geometry volumetric CT system.” Medical Physics, June 2006 33(6), pp. 1867-78). With this improvement comes the possibility of reducing planned treatment volumes (PTVs), increasing the sparing of normal tissues and increasing the dose to tumors.
Also, a large quantity of work has been accomplished to improve the ability of systems designed for image guided radiation therapy to improve patient outcome. For the case of cone-beam CT, there is a large interest in developing flat-panel detectors with improved performance (dynamic range, spatial resolution) and removing the effects of scattered x-rays reaching the detector. It has now been shown that implementing compensating filters into imaging CT systems has the potential to play a large role in reducing scatter that reaches the detector, as well as scatter within the patient delivering unnecessary patient dose.
Accordingly, there is a need for an imaging system to optimize image quality in the most clinically relevant regions of an image, while reducing dose to the patient by reducing the fluence intensity outside defined regions of interests.